range-diff: adjust the output of the commit pairs

range-diff: adjust the output of the commit pairs

This not only uses "dashed stand-ins" for "pairs" where one side is
missing (i.e. unmatched commits that are present only in one of the two
commit ranges), but also adds onelines for the reader's pleasure.

This change brings `git range-diff` yet another step closer to
feature parity with tbdiff: it now shows the oneline, too, and indicates
with `=` when the commits have identical diffs.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>